Many industries use precisely shaped parts. For example, conventional blades for use in gas turbine engines must be precisely shaped. Typically, precision investment casting or die-forging forms a metal starting blank into a blank having a blade shape.
Generally, the blank includes a tip, a root region longitudinally spaced therefrom, and a midspan region extending between the tip and the root region. A platform typically separates the root region from the midspan region. The tip may include an attached or integral shroud. A number of details must be machined into these portions of the blank, to put the precisely shaped part is in its final configuration. For example, a plurality of dove tails must be machined into the root region. These dove tails allow the blade to be secured to the rotor disk in the engine. In order to achieve acceptable engine performance, these dove tails must be formed within small tolerances. To machine the dove tails into the blank with the necessary accuracy, there must be references from which to measure the configuration of the dove tails. The references used, dictate the type of fixture that will be used to hold the blank during machining.
There are a variety of ways to provide such references. The references may be provided as taught in U.S. Pat. No. 2,577,747 issued to Gibian. Gibian teaches forging at least two hemispherical buttons or protrusions into the blank. Both buttons are disposed on the blank along the stacking axis. Using these buttons, the root region can be ground down to a predetermined thickness, and a bore can be drilled into the opposite end of the blank. Using a lathe as the fixture, the blank is held and referenced by using the thickness of the root region and the bore, allowing machining of the leading and trailing edges of the blank.
The most significant problem with the solution taught in Gibian, is that machining the bore and the thickness of root region based on the buttons will not provide sufficiently accurate results. Another problem is that although the lathe positions the blank securely along the stacking axis by using the thickness of the root region and the bore as references, the root region of the blank cannot be machined while being held in the lathe.
References may also be provided as taught in U.S. Pat. No. 3,818,646 issued to Peterson. Peterson teaches a locating button on the root portion of the blade disposed along a design axis of the blank, such as the stacking axis. A fixture utilizes this button as a reference. While in the fixture, datum planes may be ground or machined into the root and shroud portions of the blank to define accurately located surfaces for subsequent machining.
One problem with this solution is that the fixture clamps the blank along the airfoil-shaped midspan by clamping mechanisms. As a result, the fixtures are complex and therefore expensive to design, manufacture and maintain. Furthermore, clamping along the midspan makes the fixture dependent on the size and shape of the blade. Due to this dependency a number of fixtures are necessary to make all the blades in one engine, since an engine has several different size and shaped blades. Another problem with using a blade dependent fixture is that during production, time may be wasted changing between fixtures, thus significantly limiting the number of blades that can be manufactured in a period of time.
Another possible way of providing references is by encapsulating the blank in a block of material, such as a low melt alloy. The block of material is formed around the blank, so that the root region extends from the block. The sides of the block provide reference planes from which the configuration of the dove tails or other details can be determined with the necessary accuracy. Encapsulating the blank requires a complex encapsulation tool. This encapsulation tool is time consuming to design and build, is blade dependent, and is not robust. Consequently, the encapsulation tool is costly to manufacture and maintain, and a different tool is required for each differently shaped or sized blank. In addition, in this arrangement, a complex fixture is necessary to hold the block containing the blank during machining of the blank. This fixture is also expensive to design and maintain, and is blade-length dependent. Furthermore, after machining the dove tails into the blank, the material must be melted off the blank. The material must be disposed of without causing environmental problems. In addition, any impurities remaining in the blade after encapsulation may cause the blade to crack during subsequent heat treatment or operation of the engine, so removal of the impurities is critical. This removal is accomplished by acid leeching and analyzing the blades. Acid leeching is expensive and environmentally hazardous, and this in combination with the analysis of the blades, further increases the cost of producing blades using encapsulation.
Therefore, an improved blank is sought, which provides references to accurately machine details into the blank, thus forming a precisely shaped part. The fixture for use with the blank must be inexpensive to design and maintain, and easily modifiable to accommodate different size or shape blades.